EP0535523A1 - Integrated optical polarisation splitter - Google Patents

Abstract

As compared with a known integrated optical polarisation splitter composed of a straight optical directional coupler, an integrated optical polarisation splitter is intended to exhibit a shorter overall length and to impose less stringent requirements on production tolerances. This is achieved by design of the integrated optical polarisation splitter in the form of a curved rib waveguide (2) integrated on a substrate (1), with a wave-conducting layer (20) and a curved rib (21) which is formed on or over the wave-conducting layer (20) and which has a two-step profile.

Description

The invention relates to an integrated optical polarization splitter.

With such integrated optical polarization splitters, it is possible to separate mixed-polarized light, which is fed in at an input of the polarization splitter, into its two mutually independent, transversely electrically (TE) and transversely magnetically (TM) polarized field components, the two separated electromagnetic fields with mutually orthogonal polarization are available at two spatially separate outputs of the polarization splitter.

Such integrated optical polarization splitters can be implemented with optical directional couplers.

The invention specified in claim 1 is based on the object of specifying an integrated optical polarization splitter which, compared to an integrated optical polarization splitter constructed from a straight optical directional coupler, has a shorter overall length and places lower demands on manufacturing tolerances.

The functioning of the integrated optical polarization splitter according to the invention is based on the polarization selectivity of an in particular uniformly curved optical waveguide. The mixed-polarized light is coupled in at one end of the curved rib waveguide with the two-stage rib and at another end face of the rib waveguide the two mutually orthogonal polarization directions are spatially separated decoupled from each other, wherein a stage of the two-stage rib lies between the two polarization directions coupled separately from one another.

The polarization splitter according to the invention can be implemented in glasses, semiconductors or organic materials.

The polarization splitter according to the invention can be implemented in III-V semiconductor materials based on GaAs or InP, the two-stage rib being produced by epitaxial growth of a vertical layer sequence and the lateral structuring of the two-stage rib being able to be carried out by wet or dry etching processes.

A realization of the polarization splitter according to the invention in glasses is possible, for example, with the aid of the material composition SiO₂-Al₂O₃. The rib waveguide can be produced vertically here by sputtering the respective layer sequence onto the substrate. An oxidized silicon wafer is suitable as the substrate.

The wavelengths with which the polarization splitter according to the invention is operated are in the optical range.

The polarization splitter according to the invention has the following advantages:
It can be realized with a length corresponding to the length of the curved rib waveguide, which compared to the length of one with a straight integrated optical directional coupler with the same layer sequence and the same geometry parameters (polarization separation and rib height from G. Müller, L. Stoll, G. Schulte-Roth, U. Wolff "Low current plasma effect optical switch an InP", IEE Electron. Lett. 1990, Vol. 26, p.115-116) realized integrated optical Polarization splitter is only about a third. Smaller integrated optical circuits can thus be implemented with the polarization splitter according to the invention.

The component is purely passive, i.e. no complex contacting of waveguides is required, as is the case, for example, in the case of Ti-LiNbO₃-y branching devices (see J.Finak, GL Yip "An Optical TE-TM mode splitter and modulator using a Ti-LiNbO₃Y - junction waveguide", 4. Intern. Conf. On Int. Optics and Optical fiber Commm., Tokyo, 1983, Poster Session 29D3-5), which separate the polarization with the help of the electro-optical effect, is the case.

The curved rib waveguide with the two-step rib of the polarization splitter according to the invention can consist of the same vertical layer composition as a coupling and / or coupling waveguide connected to the front ends, so that the polarization splitter according to the invention and the coupling and decoupling waveguides are structured laterally in only two etching steps can. Conventional short optical directional couplers, on the other hand, require, for example, a gold layer in the coupling area (see P.Albrecht, M. Hamacher, H. Heidrich, D. Hoffmann, H.-P.Nolting, CM Weinert "TE / TM mode splitters on InGaAsP / InP", IEEE Photonics Techn. Lett. 1990, Vol. 2, pp. 114-115).

To separate the two directions of polarization, the arc length only has to be set to one step of the two-step rib on only one side of the rib. This increases the number of free parameters for optimizing the component.

The setting of the length of the curved rib waveguide can be determined for a given material and given geometry parameters by the radius of curvature of the curved rib waveguide. The radius of curvature is for uncritical tuning of the Component suitable.

The polarization splitter according to the invention can be dimensioned so that the TE mode couples over and the TM mode passes through the rib waveguide undisturbed, and so that the TM mode couples over and the TE mode passes through the rib waveguide undisturbed.

Figur 1

in einer perspektivischen Prinzipdarstellung den gekrümmt verlaufenden Rippenwellenleiter mit der zweistufigen Rippe des erfindungsgemäßen Polarisationsteilers,

Figur 2

ausschnitthaft einen Querschnitt durch den Rippenwellenleiter mit zweisufiger Rippe eines konkreten Ausführungsbeispiels des erfindungsgemäßen Polarisationsteilers, und

Figur 3

in perspektivischer Darstellung das Ausführungsbeispiel des erfindungsgemäßen Polarisationsteilers.

The invention is explained in more detail with reference to the figures in the following description. Show it:

Figure 1

the basic rib waveguide with the two-stage rib of the polarization splitter according to the invention,

Figure 2

detail of a cross section through the rib waveguide with two-stage rib of a specific embodiment of the polarization splitter according to the invention, and

Figure 3

in a perspective view the embodiment of the polarization splitter according to the invention.

In FIG. 1, the substrate is designated 1. A surface-like wave-guiding layer 20 is formed under the surface 10 of this substrate 1. A uniformly curved rib 21 with a two-stage profile is formed on the surface 10 above the wave-guiding layer 20 and, together with the wave-guiding layer 20, defines the curved waveguide 2.

The rib 21 with a two-step profile has a lower first step 210 with an outer edge 211 facing away from the center of curvature of the curved rib 21 and an inner edge 212 facing this center of curvature M and one Width W between the outer edge 211 and the inner edge 212 and also a second step 213 applied on the first step with an outer edge 214 facing away from the center of curvature and an inner edge 215 facing this center of curvature, the outer edge 214 of the second rib 213 being a distance in the radial direction r B from the outer edge 211 of the first step 210.

If you choose the jumps in refractive index n₁ and n₂ on the outer edges 211 and 214 of the first and second stages 210 and 213 and the width W of the first stage are sufficiently large, the waveguide in the rib waveguide 2 is determined by these outer edges 211 and 214 of the rib 21 alone. The influence of the two inner edges 212 and 215 of the rib 21 can be neglected in this case. As a dimensioning guide can apply: (Δn₁) ² + (Δn₂) ² is chosen very large compared to n₁²-β² and the width W of the first stage 210 is greater than r₀- (β / n₁k₀) where n₁ is the refractive index of the material of the second stage 213 , β is the phase constant of an overall field distribution of the light guided in the waveguide k₀ = 2 π / λ is the vacuum wave number and r₀ is the radius of curvature of the fin waveguide 2.

Because of the large effective jump in refractive index at the outer edges 211 and 214 of the rib 21, the radiation loss of the wave guide at these outer edges 211 and 214 is negligible.

By varying the radius of curvature r₀ of the radial distance B between the outer edges 211 and 214 and the refractive index jump Δn₁, for example at constant Δn₁ + Δn₂, a polarization splitter configuration can be set by two excessive refractive indexes on the outer edges 211 and 214 for a polarization direction, for example the TE polarization. The arc length L of the rib waveguide 2 can be approximately calculated to r₀ / (β₀-β₁), with β₀ and β₁ the phase constant of a zero and guided in the waveguide 2 first supermodes is.

A waveguide mode which is coupled in at the inner edges 212 and 215 of the ribbed waveguide 2 will emerge at the outer edges 211 and 214 of the ribbed waveguide 2 after propagation over the arc length. For the other polarization direction, in the example the TM polarization, the rib waveguide 2 is detuned due to other effective refractive indices for this polarization direction, i.e. there is no coupling of light between the outer edges 214 and 211 of the rib 21 with a two-stage profile.

Accordingly, the curved rib waveguide 2 can be used to spatially separate electromagnetic fields of different polarization directions at a distance which is determined by the distance B between the two outer edges 211 and 214 of the rib 21 with a two-stage profile.

In the specific exemplary embodiment according to FIG. 2, the substrate consists of InP with a refractive index n = 3.164. The wave-guiding layer 20 consists of GaInAsP with a gap wavelength λ _g = 1.3 μm and a refractive index n = 3.399. An etching stop layer made of GaInAsP with a gap wavelength λ _g = 1.05 μm and a refractive index n of 3.241 is applied to the wave-guiding layer 20, which defines the first step 210 with the outer edge 211. For this purpose, the etching stop layer in the area of the first step 210 has a thickness of 0.3 μm, for example, while outside the area of the step 210 it only has a thickness of 0.1 μm.

A layer of InP with a refractive index n = 3.164 is applied to the etch stop layer in the area of the rib 210, which layer has a thickness of 1.3 μm, for example, and defines the second step 213 of the rib 21 with a two-step profile. The radial distance B between the outer edge 214 and the outer edge 211 of the rib 21 is, for example, 4 μm. The radius of curvature r₀ of the ribbed waveguide 2 is, for example, 1.1 mm and its arc length L is at least 300 μm.

With the rib waveguide dimensioned in this way, the TE and TM polarization of mixed-polarized light of the wavelength λ = 1.55 μm can be spatially separated from one another, the TM-polarized light being guided essentially undisturbed in the waveguide 2, while the TE-polarized light coupled to the outer edge 211 of the first step 210 of the rib 21. For complete coupling, at least the arc length L = 300 μm of the curved rib waveguide 2 is required.

The wave-guiding layer 2 made of GaInAsP with a thickness of 0.4 μm, the etching stop layer made of GaInAsP and the layer made of InP applied thereon can be produced epitaxially in a simple manner. The etching depths given in the example according to FIG. 2 are selected according to the rules given above, the other parameters correspond to typical dimensions of the integrated optics.

FIG. 2 also shows the effective refractive index n _eff for the TE-polarized and TM-polarized light in the area under the second step 213, in the area between the outer edge 214 of the second step 213 and the outer edge 211 of the first step 210 and outside the rib 21 indicated with a two-stage profile.

FIG. 3 shows the exemplary embodiment according to FIG. 2, wherein a coupling-in waveguide 3 and two coupling-out waveguides 4 and 5 are coupled to the curved rib waveguide 2 with the rib 21 with a two-stage profile on an input-side end face 23, and two coupling-out waveguides 4 and 5, one at a time Outcoupling waveguide 4 is coupled in the area of the second stage 213 and is used for outcoupling the TM-polarized light during the other coupling-out waveguide 5 is coupled in the area of the first stage 210 and is used for coupling out the TE-polarized light spatially separated from the TM-polarized light.

The coupling-in waveguide 3 is defined by a rib 31 formed in the plane of the second stage 213. The output waveguide 4 is similarly defined by a rib 41 formed in the plane of the second stage 213. The coupling-out waveguide 5 is defined by a rib 51 formed in the plane of the first stage 210. The field distribution of the TM-polarized light guided in the coupling-out waveguide 4 is indicated by the region 40, while the field distribution of the TE-polarized light guided in the coupling-out waveguide 5 is indicated by the region 50. The width of the ribs 31, 41 and 51 can be, for example, 2 µm compared to B = 4 µm.

Claims (1)

Integrated optical polarization splitter, characterized by a curved rib waveguide (2) integrated on a substrate (1) with a waveguiding layer (20) and a curved rib (21) formed on or above the waveguiding layer (20) with two two-stage Profile.

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